1. Field of the Invention
This invention relates to sintered metal electrodes for electrochemical cells and, more particularly, to flexible sintered metal electrodes which are capable of being wound in a jelly-roll configuration for use in rechargeable electrochemical cells. Specifically, the present invention relates to improvements in such sintered metal electrodes, and in particular sintered nickel electrodes, which enhances flexibility and reduces short circuiting resulting from the winding process.
2. Description of the Prior Art
Sintered metal electrodes are used in a wide variety of electrochemical cells. In a preferred form, sintered nickel metal electrodes are highly preferred electrode forms for use both in nickel-cadmium electrochemical cells as well as nickel metal hydride cells. In typical fashion, a sintered metal electrode is formed by sintering the metal powder, such as nickel, at elevated temperatures onto a metal substrate and then impregnating the porous sintered structure with the desired active material. In the case of nickel electrodes, this active material may be in form of nickel hydroxide as well as in other forms. Other active materials may also include binders, such as polytetrafloroethylene, as well as a conductive diluents such as graphite powder or fibers or metal powder. Other metal hydroxides have also been incorporated for use as the active material and are well known in the art.
While electrodes can be formed into a number of different configurations for use in electrochemical cells, a highly utilized configuration is the jelly-roll pattern used in alkaline cells as well as rechargeable nickel cadmium and nickel metal hydride cells and the like. U.S. Pat. No. 4,929,519 and U.S. Pat. No. 4,606,982 both disclose such typical jelly-roll configurations for use in can-shaped cells. U.S. Pat. No. 5,064,735 also discloses such a configuration and the contents of this patent are specifically incorporated herein by reference.
While a wide variety of electrode types, such as sintered as well as pressed or extruded paste electrodes, are utilized in such jelly-roll configurations, sintered metal electrodes wound into such jelly-roll configurations face certain problems. One major problem is that the sintered metal electrodes tend to be relatively inflexible after sintering. Thus, when they are wound into the jelly-roll configuration, the sintered metal material with active material impregnated therein or deposited thereon tend to crack and flake. When this cracking occurs in the extreme, material can in fact spall off the substrate and fall to the bottom of the can container, thereby removing active material from use with the electrode and reduce the capacity of the cell. In less severe situations, the sintered material nonetheless flakes and tends to cause splinters. The splinters can in fact bridge the gap between electrode winds thereby causing short circuiting. This short circuiting can be intermittent or can be continuous. If enough splintering and flaking occurs, the life and capacity of the cell can be substantially shortened due to the inflexibility of the sintered metal electrode when put into a wound configuration. As a result, efforts have been ongoing to somehow increase the flexibility of the electrode in order to reduce the flaking of the sintered material and the consequent short circuiting of the formed electrode. To date, this problem has not been satisfactorily overcome in the nickel cadmium, nickel metal hydride as well as other metal electrodes used in such electrochemical cell systems.